Carbon membrane reactors CMRs

Due to their molecular-sieving effect, carbon membranes show a high H2 selectivity, and for this aspect they are considered a good candidate for 

Method Support Coating time (s) Thickness (pm) Max. pyrolysis temperature (K) Membrane position Reference 

Spinning coating Macroporous carbon disc 1 or 3 2 1073 Outside 24 

Sponge soaked + spinning coating a-Al203 modified by boehmite sol 1-3 4.7-13.9 873-973 Outside 27 

In 2000, Itoh and Haraya constructed the first CMR and experimentally examined the performance of a dehydrogenation reaction. Asymmetric polyimide hollow fibers were pyrolyzed in a vacuum oven at 1023 K in order to obtain hollow fiber carbon membranes. Their CMR consisted of SS in which 20 carbonized hollow fibers (0.295 mm diameter and 128 mm long) and catalyst pellets (0.5 wt% Pt/Al203) were allocated. The reactor, used for cyclohexane dehydrogenation to benzene at 468 K, showed a fair improvement over equilibrium conversions. In detail, the temperature dependency of the permeation rates showed that the carbon membrane had micropores with an average diameter close to those of the gas molecules and therefore the permeation process was molecular-sieving controlled. The ideal H2/Ar 

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In this chapter, we first give an overview of carbon membrane materials (Section 10.2) and the classification of carbon membranes (Section 10.3). Then, unsupported carbon membranes, based on planar membranes and asymmetric hollow fiber membranes are discussed (Section 10.4). In Section 10.5, the supported CMSMs are reviewed in detail in terms of precursors, supports, fabrications and problems. In Section 10.6, carbon-based membrane reactors are discussed in detail, based on the topics of dehydrogenation reactions, hydration reactions, hydrogen production reactions, H2O2 synthesis, bio-diesel synthesis, and new carbon membranes for carbon membrane reactors (CMRs). In the end, the new concept of using carbon membranes in microscale devices (microcarbon-based membrane reactor) is outlined (Section 10.7). 

Later, the CMRs were also used in an attempt to carry out homogeneous catalytic reactions for example, hydration of propene. Lapkin et al prepared a carbon membrane from a macroporous phenohc resin and constructed a CMR for the hydration reaction. In this gas phase continuous catalytic membrane reactor, the flat carbon membrane was used as a contactor for carrying out reactions at high temperature and pressure. In particular, the hydration of propene, catalyzed by an aqueous solution of phosphoric acid, was selected as a suitable model reaction. Olefin and water were fed separately in order to have the additional benefit of an increased alcohol concentration in the product stream because of the absence of steam in the propene feed. 

The flat carbon membranes (diameter 31 mm thickness 2-4 mm), used for carrying out experiments at 403 K and 2 MPa, were allocated in an SS reactor. The experimental results of both CMR and a conventional SLP reactor were compared using the apparent rate of production (Rp in kgaicohoi /rn membrane pore voiume h) of propan-2-ol. Rp dcpcnds oni the rate of reaction in the liquid phase the rate of gas-liquid mass transfer the mass transfer within the membrane and the convective transport in the gas space above the membrane. Therefore, Rp does not directly correspond to the intrinsic reaction rate. The best experimental measure of "Rp" was referred to equilibrium. In particular, Rp was estimated as the percentage of alcohol concentration in the vapor phase divided by the concentration of alcohol at equilibrium. 

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